The present invention relates generally to microwave oven capacity excitation systems and, more particularly, to microwave oven cavity excitation systems for promoting time-averaged uniformity of microwave energy distribution within the cooking cavity.
In a microwave oven cooking cavity, the spatial distribution of the microwave energy tends to be non-uniform. As a result, "hot spots" and "cold spots" are produced at different locations. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed. The problem becomes more severe with foods of low thermal conductivity which do not readily conduct heat from the areas which are heated by the microwave energy to those areas which are not. An example of a food falling within this class is cake. However, other foods frequently cooked in microwave ovens, such as meat, also produce unsatisfactory cooking results if the distribution of microwave energy within the oven cavity is not uniform.
A conventionally accepted explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes," are set up within the cooking cavity. When a standing wave pattern is set up, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern is dependent at least upon the frequency of microwave energy used to excite the cavity and upon the dimensions of the cavity itself. (While it is possible to theoretically predict the particular mode patterns which may be present in the cavity, it should be noted that actual experimental results are not always consistent with theory).
In an effort to alleviate the problem of non-uniform energy distribution, a great many approaches have been tried. The most common approach is the use of a device known as a "mode stirrer," which typically resembles a fan having metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material transparent to microwaves) or, to conserve space within the cooking cavity, may be mounted within a recess formed in one of the cooking cavity walls, normally the top.
The function of the mode stirrer is to continually alter the mode pattern within the cooking cavity. If a particular mode exists for only a moment, and then is immediately replaced by a mode having different hot and cold spots, then, averaged over a period of time, the energy distribution within the cavity is more uniform. In addition to varying reflection properties, a mode stirrer also tends to "pull" the oscillation frequency of the magnetron (which is a self-oscillating device) about the 2450 MHz center frequency. The cyclical variation in precise operation frequency causes different modes to be theoretically possible in the oven cooking cavity, depending also upon the precise cavity dimensions.
Another approach to the problem of non-uniform energy distribution is disclosed in commonly-assigned U.S. patent application Ser. No. 178,324, filed Aug. 15, 1980, by Matthew S. Miller, and entitled "MICROWAVE OVEN CAVITY EXCITATION SYSTEM EMPLOYING CIRCULARLY POLARIZED BEAM STEERING FOR UNIFORMITY OF ENERGY DISTRIBUTION AND IMPROVED IMPEDANCE MATCHING". The disclosed Miller microwave oven cavity excitation system introduces circularly-polarized electromagnetic wave energy into a cooking cavity through a pair of feed points appropriately phased to provide a concentrated beam. The relative phasing of the feed points is varied as a function of time to steer the concentrated beam to sweep the interior of the cavity, thereby improving the time-averaged energy distribution within the cooking cavity. Further, the disclosure of the Miller application points out that, as a result of the circular polarization, standing waves in the direction of one of the cavity dimensions are minimized, and the amount of energy reflected back to the generator is reduced. The Miller application also shows how various forms of coupling apertures or slots in a rectangular waveguide can be located with respect to the waveguide so as to radiate a circularly-polarized electromagnetic field.
From the foregoing brief summary of two approaches to achieving time-averaged uniformity of energy distribution, it will be appreciated that this is a formidable consideration in the development of practical microwave ovens.
The present invention provides a micowave energy excitation system which advantageously promotes time-averaged uniformity of microwave energy distribution within the cooking cavity.